The present invention forms part of a well established class of structures often referred to as tensegrity structures, possessing characteristics of discontinuous compression and continuous tension, where elongate members are separately placed either in tension or compression to form a self-supporting lattice. These types of structures achieve significant weight-to-strength ratios by eliminating compression members from the structural framework and replacing them with tension members wherever possible. Tensegrity structures take advantage of the high strengths that are possible for certain materials in tension and the fact that materials in pure tension do not require additional mass or added dimension in cross section to resist buckling in compression. As a result, tensegrity structures also achieve significant visual lightness as a substantial portion of the framework comprises rods, cables, or thread with diameters significantly less than the diameters or cross sectional dimensions of the compression members. A further lightness of appearance is achieved in these systems where the compression members are truly discontinuous, with no overlaps or contact, appearing to float within a continuous network of attenuated tension members.
The several continuous tension, discontinuous compression systems known to us comprise basic modules which combine to create larger structural forms, such as columns, beams, arches, planes and domes, to name a few. The basic modules of the larger aggregate structures comprise structures of their own in the form of polygons—when the compression members and the tension members are more or less co-planar—and polyhedra or prisms—when the members define the edges of a volume in space—with rigid compression struts supported in space and separated entirely from each other by a network of tension members anchored to and pulling from the ends of adjacent compression members. In these cases of existing art, the tension and compression members of the module structures are linear, an expression of the forces within the system resolved as pure axial loads. When the structural modules comprise polyhedra and prisms, the tension and, additionally or alternatively, the compression members of the structure define the linear edges and the faces of the tensegrity module.
Planar or spherical aggregates of these modules, or planar aggregates that join to create curved forms, can be construed as a lattice membrane, the weave of the membrane comprising both the continuous tension network and the discontinuous compression members, with the dimensions of the module determining the minimum thickness of the membrane. On the scale of the individual modules, however, the linear tension and compression members of the structure define the edges of a space, and the faces of the structure so defined are open and do not constitute an enclosing membrane. At scales where an aggregate of tensegrity modules is not construed as a continuous woven fabric, the tensegrity module functions as a three-dimensional frame, and additional material must be supplied to the structural framework to enclose the defined space, possibly as exterior panels or curved or triangulated infill panels attached to the tension and compression members of the structure. An alternative to panels or modular infill at the building scale is to provide enclosed volumes as independent objects nested and supported inside of the structural frame. These techniques of providing enclosure, however, are common practices utilizing many types of structural frames for support, and are not unique to frameworks comprising discontinuous elongated compression members and networks of elongated, articulated tension members.
In another example of existing art, tension membrane panels replace a number of the linear tension members of the tensegrity structure. The tension membrane panels lie in the same plane or face typically defined by the tension and compression members, or, in cases where the tension member defines the edge of two adjacent triangular faces, the tension membrane panel forms a hyperbolic paraboloid panel. The corners or apexes of these panels anchor to the ends of the compression members, pulling the ends of the members together in a fashion similar to the substituted linear tension members. With the requirement that all areas of the panel surface experience tension, the edges of these panels assume a catenary curvature between the anchor points. The panels attenuate toward the corners and assume a more or less cruxiform shape of catenary curves.
While a network of such panels can be considered a continuous tension network in the same fashion as a network of linear tension elements, substantial gaps exist between adjacent panels and between panels and compression members in examples of art utilizing this system. Consequently, the proliferation of edges renders a discontinuous surface. Further disadvantage results from the phenomena of flutter where vibrations are created by the steady flow of air or liquid over these edges.
Prior art structures utilizing a continuous tension membrane for enclosure include a class of freestanding tent. In such structures, discontinuous elongated flexible members are combined end to end to create substantially longer members. Those longer members are then used in bending to combine the properties of compression and tension to provide pre-stressing and arch support to the structure.
The invention disclosed herein, while similar in its use of a membrane to provide structure and enclosure, differs significantly from these prior art tent and related structures. As taught herein, for example, elongate members are configured purely for axial compression and not for bending. Therefore, they have significantly less flexibility and smaller ratios of slenderness. Furthermore, pre-stressing of the system does not occur as a result of energy stored in the elongate compression members through bending of the members.
It will also be known to one skilled in the art that fully enclosed structures exist wherein elongated compression members of uniform or slightly variable length radiate more or less from a common intermediate point or from several intermediate points along and generally towards the ends of a single axial compression member. The compression members of these structures support a continuous closed, anticlastic membrane with peaks corresponding to the ends of the compression members that are held in place by the uniform pull of the membrane. The intersecting compression members in such structures cannot be characterized properly as discontinuous. The resulting structures describe star-like, radiating forms of combined conical double curved surfaces, resembling complex polyhydra. Likewise, as proven out by prior art structures, each module is a closed system unto itself. The usefulness of these structures is further undermined by the density of the compression members at their centers, which effectively fills instead of creating space at the center of the enclosure. The present inventor has recognized that similar volumes of space can be achieved with more efficient use of material and space and better volume-to-weight ratios using discontinuous compression members.
Other notable examples exist in prior art wherein a continuous membrane is stretched or draped over a pre-stressed scaffolding comprising a single structural module. The scaffolding comprises a framework of discontinuous elongate members held in compression by a continuous lattice of attenuated, linear tension members. In prior art disclosures by Anne Niemetz and Andrew Pelling (The dark side of the cell, audio-visual installation, 2004, United States) and more recently by Florian Idenburg and Jing Liu (In Tension, Installation, 2010, United States) a membrane or mesh wrapper induces catenary curvature in linear tensile members in scaffolding. The membrane or mesh assumes anticlastic double curvature while contributing additional pre-stress and embodied energy to the structure. This assertedly provides increased stability.
Lasse West disclosed a freestanding display structure utilizing a single tripod of discontinuous, elongate compression members held in compression by a continuous band of tensile membrane (Trinex, Construction, 2004, Germany). The West disclosure represents a recent development in the substitution of a tensile membrane for the continuous lattice of elongated tension members in pre-stressed continuous tension, discontinuous compression structural systems. As disclosed by West, a prestressed, anticlastic membrane directs tensile forces to the ends of the discontinuous compression members as axial compressive loads. Evidencing the still further attempts of prior art inventors to devise elegant and effective tensegrity structures, Mizuki Shigematsu, Masato Tanaka and Hirohisa Noguchi have disclosed a similar module of tensegrity membrane structure based on a variational method (Form finding analysis of tensegrity membrane structures, Conference Paper, 2008, United States and Japan). Under their teachings, a single layer of a diamond-pattern system of continuous tension, discontinuous compression construction is employed. Following the prismatic model, the tensile membrane in this module engages the discontinuous compression members along their entire length. Each compression member is incorporated into the exterior surface as the edge of a linear crease. This condition induces bending loads in the compression member and requires design for bending and for axial loads yielding a resulting loss in the weight and material efficiencies of the system. The compression member must assume the function not only of a tent pole but also that of a ridge beam under uniform loads. Furthermore, the edge of the membrane at the continuous attachment to the linear compression member also panelizes the membrane, in such a way that it no longer assumes the form of a minimal surface spanning the outermost edges of the structure. This increases the surface-to-volume ratio of the structure and reduces the efficiency of material used relative to the volume of enclosed space. These shortcomings are compounded when the diamond-pattern system is utilized to create multi-layered tubular figures employing tension membranes as disclosed by Mizuki Shigematsu et al. The resulting panelization of the membrane creates discontinuities, invaginating the membrane surface while interfering with the even distribution of loads. This effectively reduces the area of the membrane engaged with loads applied to the structure.
With an awareness of the disclosures of the prior art and the shortcomings thereof, the present inventor has appreciated that there remains a need for improved tensegrity structures that exhibit desirable structural integrity while providing for efficiency in the use of material and enclosure of space and improved volume-to-weight and volume-to-surface ratios.